An electrooptical medium is a material whose refractive index and in which the speed of which light propagates vary for at least one light polarization when an electric field is applied.
The measurement of electrical voltages by means of electrooptical media is known. A corresponding appliance is disclosed, for example, in U.S. Pat. No. 4,904,931. This contains an electrooptical crystal between two polarizers. The Pockel effect causes a change in the refractive index in the crystal, and this leads to the light intensity being modulated after the second polarizer. This modulation is periodically dependent on the voltage. In order to obtain a unique measurement result, two beams at the same wavelength are therefore passed through the crystal in U.S. Pat. No. 4,904,931.
U.S. Pat. No. 4,531,092 discloses a method for measurement of an electrical voltage, in which two beams at different wavelengths are sent through an electrooptical crystal. In the process, only one of the two beams is polarized on the outside, so that the intensity of the second beam is not dependent on the voltage. This measure allows the second beam to be used as a reference variable, and allows the accuracy of the measurement to be improved.
U.S. Pat. No. 5,895,912 discloses a method for measurement of an electrical voltage by means of an electrooptical medium, with light at only one wavelength being sent through the medium. Compensation for temperature influences is achieved by means of a temperature T that is assumed to be known, and by means of signal normalization and fit functions.
U.S. Pat. No. 4,629,323 discloses a measurement apparatus for measurement of an electrical voltage by means of an electrooptical medium, with polarized light waves at two frequencies v1 V2 and with different polarization being sent through the medium. The measurement signal is converted to an electrical signal at the frequency |v1-v2|, which is phase-modulated in accordance with a phase difference which the light waves experience in the electrooptical medium. No explicit method is disclosed for correction for a voltage value which has been found, by means of a measured temperature value.
EP 0 729 033 A2 discloses an apparatus for measurement of an electrical current by means of a magnetooptical medium, with two light beams at the same wavelength passing through the medium in opposite directions. Polarizers are located in front of and behind the medium, and are used to polarize the light waves. After passing through the respective second polarizer, the light waves are detected. The influence of the temperature on a signal which is measured in this way can be separated from the influence of the electrical current that is to be measured by virtue of the non-reciprocity of the Faraday effect.
A combined temperature and voltage sensor and corresponding method are disclosed in V. N. Filippov et al., “Fiber Sensor for Simultaneous Measurement of Voltage and Temperature”, IEEE Photonics Tech. Lett. 12, 11 (2000). The voltage measurement is carried out by means of an electrooptically active crystal using two polarized light beams at different wavelengths. A λ/4 plate, which is used to produce the polarization, is used as a temperature-sensitive element, by means of which the temperature and the voltage can be determined via an equation system.
The sensor which is disclosed in the cited document by V. N. Filippov et al. forms the precharacterizing clause of patent claim 1. The method which is likewise disclosed forms the precharacterizing clause of claim 5.
For methods of the type mentioned above, the effective electrooptical coefficient must be known accurately. Since this coefficient is generally dependent on the temperature, this can lead to inaccuracies.